Power chain saws of the type under consideration include the requisite support frame for the motor and a cutter or saw bar, which bar carries an endless loop of selected saw chain mounted for displacement by means of a drive sprocket, whose configuration and pitch and appropriately correlated with the configuration and pitch of the saw chain linkages.
The aforementioned sprocket is adapted to displace the drive links of the saw chain loop which are urged into registration with the sprocket teeth as the saw chain passes around the drive sprocket having emerged from one reach of the saw bar track or channel and returning to the other reach in its endless path of travel.
The toothed sprocket component is part of an overall drive assembly that includes a centrifugal clutch mechanism which employs a clutch drum component fixedly secured to the sprocket in adjacent or abutting relation thereto and in axial alignment therewith. Both components are rotatably supported as a unit in such a manner that the output shaft of the saw chain motor can selectively engage same to rotate associated clutch drum and drive sprocket and hence impart displacement to the saw chain in its endless path of travel.
The approaches adopted by original equipment manufacturers and parts suppliers in this field include an arrangement wherein the toothed drive sprocket is fixedly secured in abutting relation to the principal wall of the clutch drum component by a brazing operation or its equivalent.
The radiating teeth of the sprocket have a substantial axial extent to accommodate lateral shifting of the saw chain during chain saw operation with the sprocket presenting an extended hub portion axially beyond the sprocket teeth to one side to register within a central aperture defined within the principal clutch drum wall whereby the clutch drum can be adequately supported in abutting relation to the end walls or surfaces of radiating teeth of the sprocket and securely anchored against axial separation.
The several end surfaces of the radiating teeth of the sprocket are separated by their intervening recesses which are open-ended. Those end surfaces of the sprocket teeth are flat and are in a common plane such that they can be drawn into abutment with the flat surface of the principal clutch drum wall and upon undergoing and appropriate brazing operation or its equivalent be secured against separation by joining over such abutment area.
Such joining provided by the brazing operation between the sprocket and clutch drum is susceptible to failure in certain circumstances. When larger torsional forces are localized at the interface between the several abutting end surfaces of the sprocket teeth and clutch drum surface as rotary motion is imparted by the sprocket teeth to the saw chain loop the joint is under substantial stress. When such forces are excessive or extreme, having regard to the relatively small area of such brazed joint, joint fatigue may occur and failure follow with the sprocket spinning off at the clutch drum surface.
Moreover, not only are twisting and shearing forces experienced during normal operations but extreme intermittent loading may occur when the saw chain is caught in the kerf stalling the chain momentarily.
Frictional forces at the face of the clutch cup and clutch shoes at the time of slippage generate substantial heat which promotes reducing resistance to fatigue contributing to ultimate failure. Further, shifting of the saw chain during chain saw operation across the surfaces of contact of respective teeth of the sprocket generate both heat and variable loading which works against the preservation of the continued unity of such a composite unit.
It has also been found, as expected, that the incidence of fatigue and joint failure of such drive assembly is much greater in larger powered chain saws because of the increased loads and likelihood of generating forces well exceeding the fatigue and failure limits.
Failure of such drive assembly in the field causes immediate shutdown of the chain saw. A replacement must be obtained which requires an inventory of such units to be maintained or the chain saw remains shut down. Lost production through shutdown inflicts a loss giving rise to the need for an improved joint for such drive assembly with sustained operating characteristics over a greater range.
This need is illustrated by referring to prior publications outlining proposals of alternatives recommended to overcome the obstacles inherent in such drive assembly arrangements.
U.S. Pat. No. 3,099,924 reveals a sprocket and drum assembly releaseably secured together so that a damaged drive sprocket may be readily replaced when required and without discarding the clutch drum component.
U.S. Pat. No. 3,144,890 discloses a sprocket wheel and associated clutch drum assembly which provides for limited axial displacement therebetween thus permitting the sprocket to accommodate shifting or side sway of saw chain as it progresses around the sprocket.
Still another alternative is described in U.S. Pat. No. 3,279,272 in which a single-piece sprocket may be brazed or otherwise secured to the clutch drum for support upon a stationary shaft and provided with suitable bearings for selective engagement with the powered output of the chain saw motor.
Other arrangements disclose frictional spring washers to accomplish engagement of the clutch drum with the sprocket wheel as outlined by U.S. Pat. No. 3,849,884 whereas in still other proposals include a slidable interconnection between sprocket wheel and clutch drum as illustrated by U.S. Pat. No. 3,519,037; or where the faces of the sprocket teeth have been pressed into the clutch drum wall by an upsetting operation as disclosed by Canadian patent No. 766,555.